1. Field of the Invention
The instant invention relates to a device for reducing the number of sample points presented to a video display device from a sampling system and more particularly to a device for reducing the number of sample points passed from an ECG processing system to a video display device while preserving the maximum and minimum values of the amplitudes of the ECG signal without causing distortion in the representative ECG signal.
2. Description of the Prior Art
In order to facilitate digital signal processing, the analog ECG signal produced by the heart and detected by an appropriate ECG detecting device is converted into a digital signal representative of the analog ECG signal. This digital signal is produced by sampling the analog ECG signal at prespecified time increments and recording the amplitude of the analog ECG signal at the particular time points. The greater the number of sample points measured per unit time, the greater the resolution of the corresponding digital signal. In particular, in order to accurately record the maximum and minimum amplitudes of the QRS portion of an signal, a high sample rate, typically 500 samples per second is used. It has been found that in the interval of one five hundredth of a second, the variation in the amplitude of the QRS signal is quite small. As a result, at a sample rate of 500 samples per second, the maximum and minimum amplitudes of the QRS signal are closely reproduced.
In typical video display devices, the graphic display screen consists of a display matrix having dimensions of 1000 pixels by 512 pixels. On such screens, it has been found useful to display about 9 seconds of ECG signals continuously on the screen for diagnostic purposes. If the EGG signal sampled at 500 samples per second is reproduced on such a display screen where the one thousand pixel axis corresponds to the time axis and if each sampled data point is reproduced on the display screen by an appropriately placed pixel, only two seconds worth of data could be displayed instead of the desired 9 seconds. Simple arithmetic shows that if 9 seconds worth of data points are to be displayed along the one thousands pixel time axis on the display screen, the number of data points presented to the display screen must be about 111 data points per second. Further arithmetic shows that if the rate of sending data points representative of the sampled ECG signal to the video display device of 500 data points per second is reduced by factor 2/9, this yields the desired display rate of about 111 data points per second. 111 data points per second will produce slightly more than 9 seconds worth of data on the display screen at one time.
FIG. 1 shows an ECG signal sampled at 500 samples per second. By comparison, FIG. 2 shows the same ECG signal sample at 111 samples per second. As can be seen, the lower sample rate produces an ECG trace with a reduced amplitude on the QRS complex.
In view of the foregoing, it is highly desirable to sample the analog ECG signal at a high sample rate such as 500 samples per second in order to accurately determine the maximum and minimum amplitudes of the QRS signals of an ECG signal. At the same time, it is highly desirable to send data points to the video display system at a reduced rate, as for example 2/9 of the sample rate of 500 data points per second, in order to present several seconds worth of sampling data on the display screen. Therefore, it is highly desirable to produce a device which converts the data collected at a high sample rate to data passed to the video display system at a lower rate while preserving an accurate representation of the originally monitored ECG signal. Specifically, it is highly desirable for the ECG trace produced by the video display system from data points passed to the video display system at a lower rate than the original sample rate of the ECG signal to accurately reproduce the maximum and minimum values of the ECG signal, particularly the QRS part of the ECG signal, despite the fact that fewer data points are used to represent the ECG signal.